/Engine/Plugins/Experimental/Mutable/Source/MutableRuntime/Public/MuR/Image.h
Calculate the number of pixels of the image, regardless of what is allocated in m_data, only using the image descriptions. For block-compressed images, it includes the wasted pixels in the blocks, in case the size is not a multiple of it in all lods. It includes the pixels in all lods
int32 CalculatePixelCount() const